The present invention generally relates to semiconductor processing, and in particular to a system and methodology for monitoring and/or controlling the fabrication of a phase shift mask.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities, there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required in integrated circuits (ICs) fabricated on small rectangular portions of the wafer, commonly known as dies. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, the surface geometry such as corners and edges of various features as well as the surface geometry of other features. To scale down device dimensions, more precise control of fabrication processes are required. The dimensions of and between features can be referred to as critical dimensions (CDs). Reducing CDs, and reproducing more accurate CDs facilitates achieving higher device densities through scaled down device dimensions and increased packing densities.
The process of manufacturing semiconductors or ICs typically includes numerous steps (e.g., exposing, baking, developing), during which hundreds of copies of an integrated circuit may be formed on a single wafer, and more particularly on each die of a wafer. In many of these steps, material is overlayed or removed from existing layers at specific locations to form desired elements of the integrated circuit. Generally, the manufacturing process involves creating several patterned layers on and into a substrate that ultimately forms the complete integrated circuit. This layering process creates electrically active regions in and on the semiconductor wafer surface. The layer to layer alignment and isolation of such electrically active regions depends, at least in part, on the precision with which features can be placed on a wafer. If the layers are not aligned properly, overlay errors can occur compromising critical dimensions and the performance of the electrically active regions and adversely affecting chip quality and reliability.
The requirement of small features with close spacing between adjacent features requires the implementation of high-resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist, and an exposing source (such as light, x-rays, or an electron beam) illuminates selected areas of the surface of the film through an intervening master template, mask or reticle for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the photoresist coating.
Light projected onto the photoresist changes properties (e.g. solubility) of the coating such that different portions thereof (e.g. the illuminated or un-illuminated portions, depending upon the type of photoresist) can be manipulated in subsequent processing steps. For example, regions of a negative photoresist become insoluble when illuminated by an exposure source such that the application of a solvent to the photoresist during a subsequent development stage removes only non-illuminated regions of the photoresist. The pattern formed in the negative photoresist layer is, thus, the negative of the pattern defined by opaque regions of the template. By contrast, in a positive photoresist, illuminated regions of the photoresist become soluble and are removed via application of a solvent during development. The pattern formed in the positive photoresist is, thus, a positive image of opaque regions on the template. Less soluble portions of the photoresist are removed in subsequent processing stages after the image has been transferred onto the wafer. The accuracy with which patterns are transferred onto the wafer is thus important to the success of the semiconductor fabrication process.
As feature sized are continually reduced, however, limitations due to the wavelength of the light utilized in semiconductor processing can adversely affect the accuracy of pattern transfers. More particularly, as feature sizes approach the wavelength of the light utilized in processing, diffraction can occur. Diffraction is a property of wave motion, in which waves spread and bend when passed through small apertures or around barriers. The pattern(s) defined within masks can contain many such small apertures and barriers, and the bending and/or spreading of the light waves is more pronounced when the size of the aperture or the barrier approximates or is smaller than the wavelength of the incoming wave. Diffraction can occur for instance where light passes adjacent an edge of a pattern formed in the mask and is scattered in multiple directions by the edge. Diffraction can lead, for example, to rounded features and/or features that do not have a desired size and/or shape. Diffraction can also result in a reduction in intensity where exposure is desired and an increase in intensity in areas where no exposure is desired.
For example, in prior art FIG. 20, a light source is directing light waves 2002 at a mask 2004. Some of the light waves 2002 pass through an aperture 2006 that is close to the size of the wavelength of the light waves 2002. The mask 2004 has been designed to develop a region 2008 on a photo resist layer 2010, so that two desired features 2012 and 2014 can be formed. The features 2012 and 2014 are desired to be rectangular, with substantially square edges. The aperture 2006 is small because the desired features 2012 and 2014 are correspondingly small.
With conventional lithography, the light waves 2002 may pass directly through the aperture 2006, exposing the region 2008, but the light waves 2002 may also be diffracted as illustrated by light waves 2016, 2018 and 2020. The diffracted wave 2016 has exposed a region 2022 and the diffracted wave 2018 has exposed a region 2024. Neither region 2022 nor region 2024 were intended to be exposed. Further, diffracted wave 2020 has exposed a triangular area 2026 on either side of the region 2008. Thus the desired feature 2014 may not have a substantially square edge due to the undesired region 2026 being exposed by the diffracted wave 2020.
Reticles known as phase shift masks can be utilized in photolithographic processing to account for diffraction. Phase shift masks facilitate compensating for the effects of diffraction which limit the precision and size to which imaged features can be reduced. The underlying concept of a phase shift mask is to selectively introduce interference and cancellation of light at portions of an image where diffraction effects may deteriorate the resolution of the image.
In lithography, resolution is typically defined as the smallest distance two features can be spaced apart while removing all photo resist between the features, and is equal to:
D=k1*(lambda/NA)
where d is the resolution, lambda is the wavelength of the exposing radiation, NA is the numerical aperture of the lens, and k1 is a process dependent constant typically having a value of approximately 0.5. While resolution may be improved by decreasing the wavelength or by using a lens with a larger NA, decreasing the wavelength and increasing the numerical aperture decreases the depth of focus (since depth of focus is proportional to lambda/NA2). In phase shift masks, features are surrounded by light transmitting regions that shift the phase of transmitted light. Masks may be constructed to shift the phase of transmitted light by varying amounts, such as 30 degrees, 60 degrees, 90 degrees, and 180 degrees. In this way, diffracted light can be effectively cancelled, resulting in a better image transfer and improved quality chips.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key cr critical elements of the invention nor delineate the scope of the invention. Its purpose is merely to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention provides a system that facilitates monitoring and/or controlling a phase shift mask fabrication process. One or more acoustic and/or light (e.g., laser) beams arc selectively directed at a mask matriculating through the fabrication process to facilitate scanning portions of the mask to detect defects, such as, for example, cracks in a quartz layer. Some of the beams pass through the mask, while other beams are reflected from the mask. Beams that are reflected from the mask can be examined to reveal information about the surface of the mask, while beams passing through the mask, such as acoustic waves, can be utilized to expose, for example, defects or other features formed below the surface of the mask.
Cracks or fractures may develop in masks, for instance, during an etching stage of the phase shift mask fabrication process and can impinge on resulting chip quality as the defect may be propagated onto the wafer during image transfer and/or may interfere with phase shifting to mitigate the adverse affects of diffraction. Controlling the mask fabrication process, such as with runtime feedback, facilitates improved mask fabrication as compared to conventional systems and thus facilitates achieving smaller feature sizes via more precise control of phase shifting of light passing through the phase shift mask.
By way of example, one or more etching components may be employed in fabricating a phase shift mask. The etching process can be monitored by comparing signatures generated from beams reflected from and/or that pass through the mask to desired signatures. By comparing desired signatures to measured signatures, runtime feedback may be employed to control the etching component and/or one or more operating parameters associated therewith, such as to adapt the etching process. For example, at the first sign(s) of a fracture, the concentration of etchants applied to the mask can be adjusted to halt and/or rectify the formation of the defect. As a result, more desirable etching can be achieved, which can in turn increase fidelity of image transfer since more precise phase shifting and the resulting interference and cancellation may thereby be facilitated.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which one or more of the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.